Ecology of North American Freshwater Fishes. Stephen T. Ross Ph. D.
Читать онлайн книгу.
Patches
Relating Assemblages to the Environment
Multivariate Statistics and Fish Assemblages
Local versus Regional Effects on Assemblages
FISHES ARE CONFRONTED by an environment that is complex and heterogeneous, with components of their habitat changing on multiple temporal and spatial scales. For instance, water temperature or turbidity might change on an hourly or daily frequency, altering the suitability of certain habitats; in contrast, substrata might change over a longer time period of days, months, years, or decades, and basic structures, such as shoreline characteristics in lakes or riffle–pool sequences in streams, might vary on a scale of many months to tens or hundreds of years.
Various approaches have been used to describe the dynamics of local habitats, albeit most have been based on terrestrial systems (Wu and Loucks 1995). While developed initially for estuarine systems, a useful approach to understanding the freshwater habitat mosaic is to view it holistically as an environment possessing both dynamic (short-term physicalchemical and biotic variability, prey fields, and predator occurrence) and static (long-term structural variability, sediment type, and shoreline context) components, each having its own influence on fishes (Peterson 2003; Peterson et al. 2007). The timing, positioning, and amount of overlap between the dynamic and the stationary components thus control the suitability of local habitats or patches.
The distinction between the terms “habitat” and “environment” has garnered considerable debate (e.g., Ryder and Kerr 1989). Considered in the static-dynamic dimensions of Peterson (2003), habitat comprises the “localized structured component that acts as a template” for organisms, whereas environment is “the sum of the biotic and abiotic surroundings, including habitat and other organisms.” Static and dynamic features are, of course, not independent, and in the long run, dynamic features can influence “static” features. For instance, the bottom type in a stream, such as a coarse gravel substratum, is a function of the interactions of stream discharge, gradient, and bed materials. In the short term, changes in water depth, velocity, temperature, and quality across a gravel bar can alter the suitability of the coarse gravel substratum for particular fish species. For instance, in the Colorado River below Glen Canyon Dam, the hypolimnial release from the dam has greatly reduced ambient water temperature as well as turbidity. Annual water temperatures now range from 9 to 14° C and turbidity is very low; prior to the dam, summer water temperatures reached to near 30° C and the river carried extensive suspended sediments (Blinn and Poff 2005). As a consequence, native fishes such as Bonytail and Humpback chubs (Gila elegans and G. cypha) are now unable to spawn in what otherwise had been favorable main channel habitats and, in fact, are largely eliminated from the main channel (Minckley et al. 2003).
LANDSCAPE ECOLOGY
Aldo Leopold, one of the founders of the conservation movement in America, developed the concept of a land ethic, espoused, for instance, in the posthumously published A Sand County Almanac (Leopold 1949). The term “landscape ecology” was first used by a German scientist, C. Troll, in 1939 (Turner 1989), and the initial development of the field took place largely in Europe where it emphasized terrestrial patterns and humans as part of the landscape (Wiens 2002; Turner 2005). The field of landscape ecology thus deals with spatial patterns, the effect of temporal and spatial scales on how organisms perceive and respond to patchiness within the environment, and linkages among the elements within the pattern.
During the period from 1940 through the 1970s, terrestrial and aquatic ecologists, by and large, continued to view their study systems as distinct units rather than as being part of an interconnected landscape, although they might infer landscape-level processes as being important in creating the ecological pattern they were studying (Turner 1989). For example, longitudinal changes in fish faunas (the patterns) were related to changes in stream sizes and gradients. The maturation of the field of landscape ecology occurred when ecologists began to study the effects of spatial patterns on ecological processes (Turner 1989). The development of the River Continuum Model, treated later in this chapter, incorporated the role of spatial patterns in watersheds on processes occurring at different points or patches in the watershed, such as changes in energy sources and functional groups of organisms, and thus represents the incorporation of a maturing view of landscape ecology.
On one level, all of us have an inherent feeling of what constitutes a landscape. A more formal, albeit terrestrially biased, definition of a landscape is “a kilometers-wide area where a cluster of interacting stands or ecosystems is repeated in similar form” (Forman and Godron 1981); it is important, however, to emphasize that landscapes can vary greatly in size. Although terrestrially focused landscape ecologists have tended to view streams and lakes as boundaries, water bodies have their own internal heterogeneity, and it is important to recognize aquatic landscapes as well as terrestrial ones (Wiens 2002). A definition more amenable to aquatic systems considers a landscape as “an area that is spatially heterogeneous in at least one factor of interest” (Turner 2005).
Terrestrial and aquatic landscapes are usually defined by the most obvious geomorphic, hydrologic, vegetational, or land-use features. Boundaries between adjacent landscapes are defined by distinct changes in spatial elements, such as a change from a riverine landscape, with patches defined by substratum and water mass characteristics, to a riparian landscape, with complex bank side vegetation, to a drier upland landscape (Figure 4.1). Landscapes are, however, interrelated so that impacts in upland and/or riparian landscapes can affect aquatic landscapes. For instance, fish species richness in the Aspen Parkland Ecoregion of Manitoba, Canada, was positively related to the quality of the terrestrial landscape within the catchment (Wilson and Xenopoulos 2008).
FIGURE 4.1. A section of the San Juan River in Utah showing the complex patterns and juxtaposition of riverine, riparian, and upland landscapes. Aquatic habitats expand during high flow, which can also restructure the streambed and adjoining wetlands. Land and river imagery provided by R. Bliesner, Keller-Bliesner Engineering.
Landscapes themselves can be grouped into regions so that a region contains several to many landscapes and is a broad geographical area that may be ecologically diverse but that has a “common macroclimate and sphere of human activity and interest” (Forman 1995). Elements within a landscape include a background matrix, patches, and corridor, all of potentially varying shapes and sizes (Forman 1995). As perceived by organisms, what is termed a patch or a corridor can vary, so that patches for one species might become part of the surrounding matrix for another. However they are defined, the recognition that habitats are themselves embedded in an expanding hierarchy of other patches, and that the recognition of patches by organisms is scale dependent, are perhaps two of the most important outcomes from the emerging field of landscape ecology (Kotliar and Wiens 1990; Grossman et al. 1995; Wiens 2002).
In small headwater